*TW*It's funny (Woody Allen/strange funny). I'd like to say it happens with absolutely no reason, but that's not accurate; my therapist taught me that. If you chain it back, you can usually figure out the why, even if the final straw was something minute.
I find that quite often there will be a number of vulnerability factors (VF's) at play that don't seem to be doing anything. Then one, almost imperceptible trigger occurs and everything instantly crumbles. Of course, I rarely realize this at the time - most often, it's in retrospect that I realize "oh yeah, this happened and that happened and then that tiny thing finally set me off."
As an example, yesterday I had a great day, but by nighttime I found myself in a horribly self-destructive, almost suicidal mood. I wasn't quite sure why at the time, but then it clicked.
- I haven't had any anti-depressant meds in a few days (combination of my fault and pharmacy's fault).
- I had the impending stress of knowing I'd see my crash photos today, after seven years.
- I'm going to be giving my termination notice soon, as I'm going back to school. Admittedly, I'm very afraid about being without full-time work (& said money), as well as the expected fears of being a total failure.
- I was trying to respond to a post where I was about to admit an irrational fear and also come to terms with the fact that I will soon be losing my therapist, whom is effectively a lifeline for me.
I won't say I'm feeling much better today, but it explained why I suddenly sank into such a deep depression after trying to reply to one post (that I never did end up posting the reply to, as of yet).
So NO reason? No. It may seem like no reason, but there is one, and probably a bunch. You just need to figure out what it is.
To answer your question though, yes, sudden and extreme mood swings over very minor things are definitely common among Borderlines. Go big or go home.